1. Field of the Invention
The present invention relates to a polysiloxane-containing rubber composition having improved properties and a rubber compounding agent therefor.
2. Description of the Related Art
Rubber compositions comprising various rubbers and a filler such as silica are known in the art and are used, for example, as rubber compositions for tire treads having a low heat generation characteristic and superior in abrasion resistance etc. However, tire treads in which silica is formulated have a low rolling resistance and a good grip on wet roads, but there were the problems that the viscosity of the unvulcanized compound increases, the vulcanization is slowed, the kneading performance in the mixing falls, etc. and the productivity deteriorates. Various proposals have been made in the past to solve these problems, but unfortunately these are not practically satisfactory.
For example, it has been proposed to add diethylene glycol or a fatty acid (for example, see Gomu Kogyo Binran (or Rubber Industry Handbook), fourth version, pp. 517 to 518, published 1994), to add a metal salt of a carboxylic acid (for example, see Tire Technology International 1995, pp. 107 to 108), and to treat the silica in advance with silicone oil (for example, see Japanese Unexamined Patent Publication (Kokai) No. 6-248116). Further, the only way to deal with scorching during mixing and a fall in the kneading performance is to increase the number of times of mixture. Further, even when mixing carbon and silica, the practice is to either mix them separately or to increase the mixing time or frequency of mixture. None of these are unfortunately practically acceptable.
On the other hand, various metal oxides are used in many fields such as rubber, cosmetics, synthetic resins, coating compositions, adhesives, magnetic tapes. In most cases, since the metal oxides are added to organic materials, problems arise how the metal oxides are dispersed in the organic materials. For this purpose; various attempts have been made to solve these problems. For example, the treatment of the metal oxides with surface treatment agents has been proposed. JP-A-8-48910 discloses a method for treating a metal oxide with methyl hydrogen polysiloxane, in which the metal oxides are necessary for treating at a high temperature. JP-A-8-53630 discloses the treatment with an alkoxy silane, but this is not still sufficient.
On the other hand, rubber compositions comprising various rubbers and metal oxides such as silica, clay, talc are known and are used as, for example, rubber compositions for tire tread having excellent low heat build-up and abrasion resistance. However, when metal oxides are compounded, there are disadvantages, when compared in the case of carbon black, the modulus is decreased and the abrasion resistance is low. To solve these problems, the above-mentioned JP-A-6-248116 has been proposed. (i.e., silica is surface-treated under heating with, for example, a hydrophobic agent such as silicone oil comprising an organic silicon compound at, for example, 250xc2x0 C. for 1 hour. However, this method has a disadvantage of a short scorching due to heating at a high temperature.
Accordingly, the objects of the present invention are to eliminate the above-mentioned disadvantageous of the prior art and to provide a filler-containing vulcanizing rubber composition which improves the processability of the unvulcanized rubber composition, without substantially impairing the properties of the filler-containing vulcanized rubber composition such as the low heat generation characteristic and the abrasion resistance.
Another object of the present invention is to provide a rubber compounding agent improving the vulcanized physical properties of a silica-containing vulcanizing rubber composition, in particular the modulus, abrasion resistance, and tans balance etc. and a silica-containing vulcanizing rubber composition using the same.
A further object of the present invention is to provide a surface-treated metal oxide capable of increasing the reinforceability when compounding into rubber and of improving the modulus and the abrasion resistance of the rubber composition and a rubber composition containing the same.
A. still further object of the present invention is to provide a silica-containing vulcanizing rubber composition for a tire tread which improves the processability of the unvulcanized rubber composition, without substantially impairing the properties of the silica-containing vulcanizing rubber composition, for example, the low heat generation characteristic and the abrasion resistance.
Other objects and advantages of the present invention will be apparent from the following description.
In accordance with the present invention, there is provided a rubber composition comprising a filler, said rubber composition containing a polysiloxane having the following alkoxysilyl group (I) and/or acyloxysilyl group (II) and having at least six alkoxy groups or at least two acyloxy groups directly bonded to the Si atom in the molecule thereof and having an average degree of polymerization of 3 to 10,000:
xe2x89xa1Sixe2x80x94OR1xe2x80x83xe2x80x83(I)
xe2x80x83xe2x89xa1Sixe2x80x94OCOR2xe2x80x83xe2x80x83(II)
wherein, R1 is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 18 carbon atoms or an organic group containing an ether bond and R2 is hydrogen or a hydrocarbon group having 1 to 21 carbon atoms.
In accordance with the present invention, there is provided a rubber compounding agent comprising (A) a polysiloxane having the following alkoxysilyl group (I) and/or acyloxysilyl group (II) and having an average degree of polymerization of 3 to 10,000 and (B) a silane coupling agent in a ratio of (A)/(B)=95/5 to 5/95:
xe2x89xa1Sixe2x80x94OR1xe2x80x83xe2x80x83(I)
xe2x89xa1Sixe2x80x94OCOR2xe2x80x83xe2x80x83(II)
wherein, R1 is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 18 carbon atoms or an organic group containing an ether bond and R2 is hydrogen or a hydrocarbon group having 1 to 21 carbon atoms.
In accordance with the present invention, there is further provided a rubber composition comprising 100 parts by weight of a starting rubber, 5 to 100 parts by weight of silica, and an amount of the above-mentioned rubber compounding agent to give a content of the polysiloxane contained therein of 0.2 to 30% by weight in the total composition.
In accordance with the present invention, there is further provided a polysiloxane surface-treated metal oxide comprising 100 parts by weight of a metal oxide surface treated with 0.1-50 parts by weight of a polysiloxane having the following alkoxysilyl group (I) and/or acyloxysilyl group (II) and having an average degree of polymerization of 3 to 10,000:
xe2x89xa1Sixe2x80x94OR1xe2x80x83xe2x80x83(I)
xe2x89xa1Sixe2x80x94OCOR2xe2x80x83xe2x80x83(II)
wherein, R1 is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 18 carbon atoms or an organic group containing an ether bond and R2 is hydrogen or a hydrocarbon group having 1 to 21 carbon atoms.
In accordance with the present invention, there is further provided a rubber composition comprising 100 parts by weight of a starting rubber and 5 to 100 parts by weight of the above-mentioned polysiloxane surface-treated metal oxide.
In accordance with the present invention, there is provided a rubber composition for a pneumatic tire tread comprising 100 parts by weight of a diene rubber, 2 to 80 parts by weight of carbon black, 5 to 80 parts by weight of silica, a silane coupling agent, and a polysiloxane having the following alkoxysilyl group (I) and/or acyloxysilyl group (II) and having an average degree of polymerization of 3 to 10,000:
xe2x89xa1Sixe2x80x94OR1xe2x80x83xe2x80x83(I)
xe2x89xa1Sixe2x80x94OCOR2xe2x80x83xe2x80x83(II)
wherein R1 and R2 are the same as defined above.
The term xe2x80x9cfillerxe2x80x9d used herein means any conventional inorganic fillers (e.g., calcium carbonate, clay, talc, diatomaceous silica, mica, alumina, aluminum sulfate, barium sulfate, calcium sulfate, etc.) and so-called reinforcing agents, (e.g., carbon black, silica, etc.). The term xe2x80x9csilicaxe2x80x9d used herein means wet silica and dry silica and silica having a nitrogen specific surface area of 50 to 400 m2/g. The use of the wet silica is preferable. Further, the term xe2x80x9cvulcanizationxe2x80x9d used herein includes cross-linking by sulfur, peroxide, etc. in addition to the usual vulcanization by sulfur.
The first aspect of the present invention provides a rubber composition comprising a filler, said rubber composition containing a polysiloxane having the following alkoxysilyl group (I) and/or acyloxysilyl group (II) and having an average degree of polymerization of 3 to 10,000, preferably 10 to 1,000:
xe2x89xa1Sixe2x80x94OR1xe2x80x83xe2x80x83(I)
xe2x80x83xe2x89xa1Sixe2x80x94OCOR2xe2x80x83xe2x80x83(II)
wherein, R1 is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 18 carbon atoms, preferably an alkyl group having 1 to 6 carbon atoms and an aryl group having 6 to 12 carbon atoms, or an organic group containing an ether bond, preferably R6(OR7 )n (wherein R6 is C1-C6 alkyl, R7 is C2-C4 alkylene, n is 1 to 3, R2 is hydrogen or a hydrocarbon group having 1 to 21 carbon atoms, preferably C1-C21 alkyl or C6-C21 aryl which may be substituted with C1-C6 alkyl. More preferably, the polysiloxane has at least six alkoxy groups and/or at least two acyloxy groups directly bonded to the Si atom in the molecule thereof.
According to a preferable embodiment of the present invention, there is provided a rubber composition further containing in the rubber composition a silane coupling agent in an amount of 40% by weight or less of the silica content.
As explained above, there was the problem that the vulcanized properties of the tire tread containing silica were excellent, but the processability at the time when unvulcanized was poor. We have found that, this is due to the silanol groups (Sixe2x80x94OH) present on the surface of the silica. The cohesiveness of the silanol groups causes the production of structures in the rubber composition and the increase in the viscosity, the polarity of the silanol groups causes the vulcanization accelerator etc. to be adsorbed and vulcanization to be delayed, and the compatibility with the nonpolarized rubber is insufficient, and therefore, the kneading performance in the mixing is decreased. Due to these phenomena, the processability of the unvulcanized composition falls. Further, in many cases silane coupling agents can be jointly used in a silica-containing rubber composition to reinforce the rubber, but silanol groups are also present inside the silica particles and there was the problem that these would react with the silane coupling agent to cause a loss of the silane coupling agent and reduce the reinforcing effect, making inclusion of a large amount of a silane coupling agent necessary. When diethylene glycol or other polarized substance is added to this, it is possible to prevent to a certain extent the phenomenon of adsorption of the vulcanization accelerator or other polarized compounding agents, but complete prevention is not possible and it was not possible to prevent the substances chemically bonding with a silane coupling agent or other silica particles from bonding inside.
According to the present invention, since a polysiloxane having the alkoxysilyl groups and/or acyloxysilyl groups of the above formulae (I) and/or (II), respectively, is blended into the rubber composition, the alkoxysilyl groups (I) and/or acyloxysilyl groups (II) react with the silanol groups and covers the surface of the silica particles, and therefore, the problems in the prior art are solved and it is possible to effectively suppress the rise in viscosity caused by the cohesiveness and polarity of the silanol groups and the wasted consumption of the vulcanization accelerator or other polarized additives or silane coupling agent etc.
The polysiloxane containing the alkoxysilyl groups (I) and/or acyloxysilyl groups (II) blended in the rubber composition according to the present invention, as mentioned above, must have an alkoxysilyl group (I) and/or acyloxysilyl group (II) reacting with a silanol group and be a polymer (or oligomer) of a size covering the surface of the silica particles and exhibit a lubricating effect, for example, an average degree of polymerization of 3 to 10,000, preferably 10 to 1,000 (or a number average molecular weight of 200 to 300,000, preferably 500 to 50,000). Accordingly, in the polysiloxane of the present invention, it is essential that a xe2x89xa1Sixe2x80x94Oxe2x80x94R3 group and/or xe2x89xa1Sixe2x80x94OCOR2 group be present. These groups may be at the main chain, side chains, or ends. In addition, the presence of at least six alkoxy groups and/or at least two acyloxy groups preferably directly bonded to the Si atom in one molecule thereof provides the strong bonding with the filler. Further, a hydrogen group or other organic groups are also possible. The polysiloxane usable in the first aspect of the present invention preferably has at least one hydrocarbon group such as alkyl group directly bonded to the Si atom in the molecule thereof in view of the affinity thereof with the rubber component.
The polysiloxane is a known substance. For example, it may be manufactured as follows:
The polysiloxane containing an alkoxysilyl or acyloxysilyl group is synthesized by causing a reaction between an Sixe2x80x94H group containing polysiloxane and alcohol or carboxylic acid in the presence of a catalyst.
As the xe2x89xa1Sixe2x80x94H group containing polysiloxane, the ones illustrated below may be mentioned: 
As the alcohol usable in the present invention, methanol, ethanol, propanol, butanol, pentanol, heptanol, octanol, octadecanol, phenol, benzyl alcohol, and also ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, and other alcohols having oxygen atoms may be illustrated.
As the carboxylic acid, acetic acid, propionic acid, palmitic acid, stearic acid, myristic acid, etc. may be mentioned.
As the catalyst, chloroplatinic acid, platinum-ether complexes, platinum-olefin complexes, PdCl2 (PPh3)2, RhCl2 (PPh3)2, or basic catalysts may be used. The corresponding xe2x89xa1Sixe2x80x94H group containing polysiloxane and alcohol or carboxylic acid are reacted in the presence of the catalyst for synthesis.
As the method for introducing the organic group, introduction is easy by causing a reaction of xe2x89xa1Sixe2x80x94H and an organic compound having a double bond. As a compound having a double bond, there are styrene, xcex1-methylstyrene, xcex1-methylstyrene dimer, limonene, vinylcyclohexene, etc.
As another method, synthesis is possible by causing a reaction between a corresponding xe2x89xa1Sixe2x80x94H group containing polysiloxane and a double bond containing alkoxysilane as shown below in the presence of the above catalyst: 
As still another method, the polysiloxane used in the present invention may be synthesized by causing a reaction between a silanol terminal polysiloxane and an alkoxysilane in the presence of a bivalent tin compound or other catalyst. Examples of such a silanol terminal polysiloxane are: 
wherein n is 1 to 2000
Examples of the alkoxysilane, are the following alkoxysilanes. Further, the silane coupling agents shown in Table I are exemplified. 
The polysiloxane usable in the present invention may further be synthesized by a reaction between polysiloxane having a reactive functional group at its side chain or terminal and a silane coupling agent of Table I. Examples of the polysiloxane having a reactive functional group, are an epoxy group, amine group, mercapto group, carboxyl group, etc.
Note that the polysiloxane used in the present invention, as explained above, is not particularly limited in its terminal groups and side chains and is determined by the type of the starting material used during manufacture.
The amount of the polysiloxane used in the first aspect of the present invention is preferably 100% by weight or less, preferably 1 to 100% by weight, particularly preferably 2 to 40% by weight, of the weight of the filler in the rubber composition. When the content of the polysiloxane is too small, the desired effect cannot be obtained, while conversely if too great, substances not bonding with the filler (e.g., silica) will leak out from the vulcanized product in some cases, which is not desired.
Although there are no specific limitations to the amount of the filler to be kneaded, preferably 5 to 150 parts by weight, more preferably 20 to 120 parts by weight, particularly preferably 50 to 90 parts by weight of the filler based upon 100 parts by weight of the rubber component can be used.
The rubber contained as the main ingredient in the vulcanizable rubber composition according to the present invention may be any rubber generally contained in various rubber compositions in the past, for example, natural rubber (NR), polyisoprene rubber (IR), various styrene-butadiene copolymer rubbers (SBR), various polybutadiene rubbers (BR), acrylonitrile-butadiene copolymer rubbers (NBR), butyl rubber (IIR), and other diene rubbers or ethylene-propylene copolymer rubbers (EPR, EPDM) etc. alone or as any blends.
The vulcanizable rubber composition according to a preferred mode of the second aspect of the present invention further contains a silane coupling agent. The silane coupling agent used in the present invention may be made any silane coupling agent used together with silica fillers in the past. The typical examples are shown in Table I. Of these, bis-[3-(triethoxysilica)-propyl ]tetrasulfide and the silane coupling agents shown in Table Ixe2x80x2 below is most preferred from the viewpoint of the processability.
When a silane coupling agent is mixed into the vulcanizing rubber composition according to the present invention, it is possible to reduce the amount of the silane coupling agent used compared with the past and it is possible to further improve the abrasion resistance. The preferable amount of the silane coupling agent used in the present invention is 40% by weight or less, preferably 0.5 to 40% by weight, particularly preferably 1 to 20% by weight, based upon the amount of the silica in the composition. When the amount of the silane coupling agent is too small, the desired effects cannot be obtained, while conversely when too great, scorching will easily occur in the mixing or extrusion step, which is not desirable.
According to the second aspect of the present invention, there is provided a rubber compounding agent including (A) a polysiloxane containing the following alkoxysilyl group (I) and/or acyloxysilyl group (II) and having an average degree of polymerization of 3 to 10,000 and (B) a silane coupling agent in a ratio of (A)/(B)=95/5 to 5/95:
xe2x89xa1Sixe2x80x94OR1xe2x80x83xe2x80x83(I)
xe2x89xa1Sixe2x80x94OCOR2xe2x80x83xe2x80x83(II)
wherein R1 and R2 are the same as explained above.
According to the second aspect of the present invention, there is provided a rubber compounding agent wherein the polysiloxane (A) is impregnated in (C) at least one powder in a ratio of (A)/(C)=70/30 to 5/95.
According to the second aspect of the present invention, there is provided a rubber composition comprising 100 parts by weight of the starting rubber, 5 to 100 parts by weight of silica, and an amount of the rubber compounding agent composed of (A) and (B) or (A) and (C) to give a content of the polysiloxane contained therein of 0.2 to 30% by weight in the total composition.
As mentioned above, if a polysiloxane (A) having an alkoxysilyl group or acyloxysilyl group of the formula (I) or (II) is blended in a rubber composition, the alkoxysilyl groups (I) or the acyloxysilyl groups (II) react with the silanol groups and cover the surface of the silica particles, and therefore, the problems in the prior art are solved and it is possible to effectively suppress the rise in viscosity caused by the cohesiveness and polarity of the silanol groups and the wasted consumption of the vulcanization accelerator or other polarized additives or silane coupling agent etc. However, the silane coupling agent for increasing the reinforcing nature of the silica and the polysiloxane for improving the processability of the silica both react with the silanol groups on the surface of the silica in competitive reactions. Thus, we found that the physical properties of the rubber differed depending on the method of mixture (or order). That is, if the silica and polysiloxane react first, the reinforcing nature falls, which is not desirable. Therefore, in the present invention, the coupling agent and polysiloxane are mixed in advance into the rubber, so reaction of just the polysiloxane first is prevented or the polysiloxane is pre-impregnated into carbon black or another nonreactive filler (i.e., inert powder) or silica or another powder so as to delay the reaction with the silica and thereby prevent a decline in the vulcanized physical properties of the rubber.
The polysiloxane (A) containing the alkoxysilyl groups (I) or acyloxysilyl groups (II) blended in the rubber composition according to the present invention, as mentioned above, must have an alkoxysilyl group (I) and/or acyloxysilyl group (II) reacting with a silanol group and be a polymer (or oligomer) of a size covering the surface of the silica particles and exhibit a lubricating effect, for example, an average degree of polymerization of 3 to 10,000, preferably 10 to 1,000. Accordingly, in the polysiloxane (A) used in the present invention, it is essential that a xe2x89xa1Sixe2x80x94Oxe2x80x94R1 group or xe2x89xa1Sixe2x80x94OCOR2 group be present. These groups may be at the main chain, side chains, or ends. Further, a hydrogen group or other organic group is also possible. The polysiloxane is a known substance, as mentioned above.
The polysiloxane (A) used in the present invention may further be synthesized by a reaction between polysiloxane having a reactive functional group at its side chain or terminal and a silane coupling agent of Table I. As the polysiloxane having a reactive functional group, mention may be made of an epoxy group, amine group, mercapto group, carboxyl group, etc., also as mentioned above.
Note that the polysiloxane (A) used in the present invention, as explained above, is not particularly limited in its terminal groups and side chains and is determined by the type of the feedstock used during manufacture.
The polysiloxane (A) used in the present invention is mixed in to give 0.2 to 30% by weight, preferably 1.0 to 10% by weight, based upon the rubber composition. If the content of the polysiloxane (A) is too small, the desired effect cannot be obtained, while conversely if too great, substances not bonding with the silica will leak out from the vulcanized product in some cases, which is not desired.
The rubber contained as the main ingredient in the vulcanizing rubber composition according to the present invention may be any rubber generally contained in various rubber compositions in the past, as mentioned above.
The silane coupling agent (B) used in the vulcanizing rubber composition of the present invention may be made any silane coupling agent used together with silica fillers in the past. As typical examples, any agent shown in Table I may be mentioned. of these, bis-[3-(triethoxysilica)-propyl]tetrasulfide and the silane coupling agents shown in Table Ixe2x80x2 below is most preferred from the viewpoint of the processability.
If a silane coupling agent (B) is mixed into the vulcanizing rubber composition according to the present invention, it is possible to reduce the amount of the silane coupling agent (B) used compared with the past and it is possible to further improve the abrasion resistance. The preferable amount of the silane coupling agent (B) used in the present invention is, in terms of a ratio (ratio by weight) of the polysiloxane (A) in the composition and the silane coupling agent (B) of 95/5 to 5/95, particularly preferably 60/40 to 80/20. If the amount of the silane coupling agent (B) is too small, the desired effects cannot be obtained, while conversely if too great, scorching will easily occur in the mixing or extrusion step, which is not desirable.
The rubber composition of the second embodiment according to the second aspect of the present invention is composed of at least one powder (C) generally mixed with rubber compositions in the past mixed with the polysiloxane (A). Examples of the powder (C) are carbon black, calcium carbonate, stearic acid, and other inert powders, silica, etc. Here, xe2x80x9cinert powderxe2x80x9d means a powder with a small reactivity with the polysiloxane (A) of the present invention. The amount of this inert powder (C) is, in terms of the ratio of weight of (A)/(C), 70/30 to 5/95, more preferably 60/40 to 30/70. If the amount of the inert powder is too small, the silica and polysiloxane will react overly fast, whereby the reinforcing property will fall, which is not desirable, while conversely when too great, the effect of surface treatment of the polysiloxane on the silica will become smaller, which is also not desirable. The method of impregnation of the polysiloxane (A) to the powder (C) is not particularly limited, but use may be made of a mixer, kneader, mill, etc. usually used according to need.
The silica-containing rubber composition according to the present invention is composed of 5 to 100 parts by weight, preferably 5 to 80 parts by weight, of rubber use silica based upon 100 parts by weight of the starting rubber and an amount of the rubber compounding agent (that is, the premix of the polysiloxane (A) and the silane coupling agent (B) or powder (C)) so as to give 0.2 to 30% by weight, preferably 1 to 10% by weight, of polysiloxane (A) in the total composition.
The third aspect of the present invention provides a polysiloxane surface-treated metal oxide comprising 100 parts by weight of a metal oxide surface treated with 0.1-50 parts by weight of a polysiloxane having the following alkoxysilyl group (I) or acyloxysilyl group (II) and having an average degree of polymerization of 3 to 10,000:
xe2x89xa1Sixe2x80x94OR1xe2x80x83xe2x80x83(I)
xe2x89xa1Sixe2x80x94OCOR2xe2x80x83xe2x80x83(II)
wherein R1 and R2 are the same as explained above.
According to preferable embodiments of the third aspect, there is provided a polysiloxane surface-treated metal oxide, wherein the metal oxide is SiO2 or SiO2xe2x80x94 containing metal oxide or wherein 0.05 to 50% by weight of a titanium catalyst, based upon the amount of the polysiloxane used, is further used during the surface treatment.
According to the third aspect, there is also provided a rubber composition comprising 100 parts by weight of a starting rubber and 5 to 100 parts by weight of the above-mentioned polysiloxane surface-treated metal oxide, preferably silica and optionally a silane coupling agent in an amount of 0.5-40% by weight of the amount of the silica is further contained in the composition.
As mentioned above, the polysiloxane containing the alkoxysilyl groups (I) or acyloxysilyl groups (II) according to the present invention, as mentioned above, must have an alkoxysilyl group (I) or acyloxysilyl group (II) reacting with a silanol group and be a polymer (or oligomer) of a size covering the surface of the silica particles and exhibit a lubricating effect, for example, an average degree of polymerization of 3 to 10,000, preferably 10 to 1,000. Accordingly, in the polysiloxane of the present invention, it is essential that a xe2x89xa1Sixe2x80x94Oxe2x80x94R1 group or xe2x89xa1Sixe2x80x94OCOR2 group be present. These groups may be at the main chain, side chains, or ends. Further, a hydrogen group or other organic group is also possible. The polysiloxane is a known substance and can be synthesized by the reaction between an Sixe2x80x94H group containing polysiloxane and alcohol or carboxylic acid in the presence of a catalyst, as mentioned above.
The polysiloxane usable in the present invention may further be synthesized by a reaction between polysiloxane having a reactive functional group at its side chain or terminal and a silane coupling agent of Table I shown above. Examples of the polysiloxane having a reactive functional group, are an epoxy group, amine group, mercapto group, carboxyl group, etc.
Note that the polysiloxane used in the present invention, as explained above, is not particularly limited in its terminal groups and side chains and is determined by the type of the starting material used during manufacture. According to the present invention, the surface of the metal oxide is surface treated with the polysiloxane. The surface treating method is not specifically limited. Generally speaking, the metal oxide can be impregnated or coated with the polysiloxane at room temperature in an appropriate solvent (e.g., acetone, methanol, ethanol), followed by heat drying at room temperature to 120xc2x0 C.
The amount of the polysiloxane to be used for the surface treatment according to the present invention is generally 0.1 to 50 parts by weight, preferably 1 to 20 parts by weight. If the amount of the polysiloxane is too small, the desired results cannot be obtained, while conversely it too large, the polysiloxane which does not react with the metal oxide is unpreferably exdudated from the vulcanized product.
When the surface-treated metal oxide is added in an amount of 5-100 parts by weight, preferably 10-80 parts by weight, to 100 parts by weight of the starting rubber, the desired reinforcing effects can be obtained. When the amount is less than 5 parts by weight, the desired effects of the present invention are not sufficient, while more than 100 parts by weight, the processability unpreferably becomes poor.
Examples of the metal oxides according to the present invention are metal oxides comprising a single metal such as silicon oxide, titanium oxide, aluminum oxide, iron oxide, zirconium oxide, cerium oxide and complex or composite oxides such as calcium silicate, aluminum silicate, magnesium silicate, zeolite, feldspar, kaolinite, clay, talc. Here, as the metal oxides conventionally used for rubber compounding, silicate fillers such as silicic anhydride, silicic hydrate, calcium silicate, aluminum silicate, kaolin, talc are preferable.
The titanium catalysts usable in the preferable embodiment of the present invention are titanium compounds such as alkoxy titanium, titanium chelate, titanium acylate, complex or composite titanate. As typical examples, the titanium compounds TA-10 and TC-100 (available from Matsumoto Koushou K. K. Japan) having the following structure can be effectively used. 
In the preferable embodiment of the third aspect of the present invention, the above titanium catalyst is used in an amount of 0.05 to 50% by weight, more preferably 0.1 to 10% by weight, of the amount of the polysiloxane. If the amount is too small, the desired effect of the surface treatment cannot be efficiently obtained, while conversely too large, the processability tends to be decreased when the rubber composition is prepared.
The rubber contained as the main ingredient in the vulcanizable rubber composition according to the present invention may be any conventional rubber as mentioned previously.
The vulcanizable rubber composition according to a preferred mode of the third aspect of the present invention may further contains a silane coupling agent. The silane coupling agent usable in the present invention may be any silane coupling agent used together with silica fillers in the past. The typical examples, are shown in Table I. Further, the following special sulfur-containing silane coupling agents shown in Table Ixe2x80x2 below can also be used. Of these, bis-[3-(triethoxysilica)-propyl]tetrasulfide is most preferred from the viewpoint of the processability.
When a silane coupling agent is mixed into the vulcanizing rubber composition according to the third aspect of the present invention, it is possible to improve the abrasion resistance. The preferable amount of the silane coupling agent usable in the present invention is 1 to 20% by weight, preferably 2 to 10% by weight, based upon the amount of the silica in the composition. When the amount of the silane coupling agent is too small, the desired effects cannot be obtained, while conversely when too large, scorching will easily occur in the mixing or extrusion step, which is not desirable.
According to the fourth aspect of the present invention, there is provided a rubber composition for a tire tread including 100 parts by weight of a diene rubber, 2 to 80 parts by weight, preferably 5 to 60 parts by weight, of carbon black, 5 to 80 parts by weight, preferably 10 to 50 parts by weight, of silica, a silane coupling agent, and the polysiloxane mentioned above. Examples of such a polysiloxane has repeating units of the formula (III): 
wherein, R3 represents independently a methyl group, ethyl group, or phenyl group, R4 indicates independently hydrogen or an organic group (e.g., methyl, phenylethyl, 2-(4-methyl-3-cyclohexenyl)-propyl, 2,4-diphenyl-4-methylpentyl, R5 indicates independently an alkyl group or acyl group, m is 0 or an integer of 1 or more, and n is an integer of 1 or more.
In accordance with a preferred embodiment the fourth of the present invention, there is provided a rubber composition for a tire tread as mentioned above, wherein the amounts of the polysiloxane and silane coupling agent are:
0.5xe2x89xa6(Wps/Wsc)xe2x89xa67
Content of silica xc3x971 wt %xe2x89xa6Wps+Wscxe2x89xa6
Content of silica xc3x9740 wt %
wherein, Wps: content of polysiloxane (parts by weight), and Wsc: content of silane coupling agent (parts by weight).
According to a preferred embodiment of the fourth aspect of the present invention, there is provided a rubber composition for a tire tread as mentioned above, wherein the compound except for the vulcanization system is obtained by mixing at a temperature of at least 120xc2x0 C. in a simultaneous step.
As explained above, there was the problem that the vulcanized properties of the tire tread containing silica were excellent, but the processability at the time when unvulcanized was poor.
However, according to the present invention, since a polysiloxane having the repeating structural units of the above formula (III) is blended into the rubber composition, the alkoxysiloxane reacts with the silanol groups and covers the surface of the silica particles, and therefore, the problems in the prior art are solved and it is possible to effectively suppress the rise in viscosity caused by the cohesiveness and polarity of the silanol groups and the wasted consumption of the vulcanization accelerator or other polarized additives or silane coupling agent etc.
The polysiloxane of the above formula (III) contained in the rubber composition according to the present invention, as mentioned above, must have an alkoxysilyl group or acyloxysilyl group reacting with a silanol group and be a polymer (or oligomer) of a size covering the surface of the silica particles and exhibit a lubricating effect, for example, an average degree of polymerization of 3 to 10,000, preferably 10 to 1,000. Accordingly, in the repeating unit of the above formula (III), it is essential that a xe2x89xa1Sixe2x80x94Oxe2x80x94R3 group be present. Accordingly, n should be at least 1, preferably 5 to 1000 and m may be zero, but a hydrogen group or other organic group is also possible. The polysiloxane is a known substance. For example, it may be manufactured as follows:
The compound having the siloxane structure of formula (III) is synthesized by causing a reaction between the corresponding polyalkylhydrogensiloxane and alcohol or carboxylic acid in the presence of a catalyst. Examples of the polyalkylhydrogensiloxane are those mentioned above.
The polysiloxane used in the present invention, as explained above, is not particularly limited in its terminal groups and is determined by the type of the feedstock used during manufacture. For example, it may have a trimethylsilyl group, methyldiphenylsilyl group, triphenylsilyl group, and also organic groups.
In formula (III), as mentioned above, R1 represents a methyl group, ethyl group, or phenyl group, and R2 represents hydrogen or an organic group, for example, CH3, C2H5), styrene residue, divinylbenzene residue, limonene residue, butadiene residue, isoprene residue, etc.
The diene rubber contained as the main ingredient in the vulcanizing rubber composition according to the present invention may be any diene rubber generally contained in various rubber compositions as mentioned above.
The silane coupling agent used in the present invention may be made any silane coupling agent used together with silica fillers in the past. Typical examples are shown in Table I above. Of these, bis-[3-(triethoxysilica)-propyl]tetrasulfide is most preferred from the viewpoint of the processability.
The amounts of the polysiloxane and silane coupling agent used in the present invention are a total weight of the polysiloxane and silane coupling agent of 0.5 to 40% by weight, preferably 1 to 20% by weight based upon the amount of the silica, and a weight ratio (of polysiloxane/silane coupling agent) of 0.5 to 7, preferably a range of 1 to 4. If the amount of the polysiloxane is too small, the desired effects cannot be obtained, while conversely if too great, the substances not bonding with the silica will leak out from the vulcanized product in some cases, which is not desirable.
Further, if the amount of the silane coupling agent is too small, the desired effects cannot be obtained, while conversely if too great, scorching will easily occur in the mixing or extrusion step, which is not desirable.
Note that when the silica and carbon black are mixed in the same step in accordance with the preferred embodiment of the present invention, sufficient contact and reaction occur between the diene rubber and carbon black and the mixing is promoted, but sufficient contact and reaction are not obtained between the diene rubber and silica due to the polarity of the silanol groups on the surface of the silica and therefore knitting, scorching, etc. occur at the time of mixture and the dispersion also becomes poor.
Further, when separately mixing the silica and carbon black, for example, when mixing the silica in the first step and adding the carbon black and other compounding agents in the second step, there is not sufficient promotion of mixture of the diene rubber and carbon black in the second step and it is difficult to obtain the inherent reinforcement of the carbon black in some cases. As mentioned above, therefore, according to the present invention, by adding a silane coupling agent and the polysiloxane shown in formula (I), the knitting and scorching at the time of mixing in a simultaneous step are improved and the dispersion promoted and it is possible to obtain the same or better performance as the case of mixing the silica and carbon by separate steps.
The rubber composition according to the first aspect of the present invention may contain, in addition to the above-mentioned essential ingredients, a vulcanization or cross-linking agent, a vulcanization or cross-linking accelerator, various types of oils, an antiaging agent, reinforcing agent, plasticizer, softening agent, or other various additives generally mixed into general rubbers. The compounds are kneaded and vulcanized by general methods to make the composition which may then be used for vulcanization or cross-linking The amounts of these additives added may be made the amounts generally added in the past so long as they do not run counter to the object of the present invention.